I have scatterplots with date/time along the X axis, and data on the Y axis.  
I'd like to add shading from sunrise to sunset.  

I can create a line on the scatterplot that is 0 during the day and a value
greater than the max Y value at night, but would like to be able to shade
under this line to more easily visualize when it's dark.

I also tried creating a bar graph in the background on a second X axis,
thinking I could have small categories with date and light/dark data, but the
time axis cannot do increments smaller than days, so that hasn't worked.

Any suggestions?  I have hundreds of these graphs that I'd like to add this
data to.

Thanks